Variable diameter piston assembly for safety valve

ABSTRACT

Disclosed are subsurface safety valves having a telescoping piston assembly. One disclosed safety valve includes a housing defining a piston bore that provides an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, and a piston assembly movably arranged within the piston bore and comprising a piston rod that includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion, wherein the piston dynamically seals an inner wall of the upper bore section when the piston assembly moves within the piston bore and dynamically seals an outer surface of the upper portion of the piston rod when the piston engages the bore shoulder and the piston rod continues moving.

BACKGROUND

The present disclosure relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in particular, to subsurface safety valves having a telescoping piston assembly to increase the opening force for the safety valve.

Subsurface safety valves are well known in the oil and gas industry and act as a failsafe to prevent the uncontrolled release of reservoir fluids in the event of a worst-case scenario surface disaster. Typical subsurface safety valves are flapper-type valves that are opened and closed with the help of a flow tube moving linearly within the production tubular. The flow tube is often controlled hydraulically from the surface and is forced into its open position using a piston and rod assembly that may be hydraulically charged via a control line linked directly to a hydraulic manifold or control panel at the well surface. When sufficient hydraulic pressure is conveyed to the subsurface safety valve via the control line, the piston and rod assembly forces the flow tube downwards, which compresses a spring and simultaneously pushes the flapper downwards to the open position. When the hydraulic pressure is removed from the control line, the spring pushes the flow tube back up, which allows the flapper to move into its closed position.

Depending on the size and depth of the safety valve deployed, the components of the pressure control system used to operate the safety valve can be quite expensive. The cost of a pressure control system may increase as required pressure ratings for the control line and/or the pump equipment increase, which is usually related to the operating depth of the safety valve. There are practical limits to the size and rating of pressure control systems, past which a well operator may not be able to economically or feasibly employ a subsurface safety valve. Accordingly, there is always a need in the industry for the ability to use lower rated pressure control systems for operating subsurface safety valves.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, without departing from the scope of this disclosure.

FIG. 1 is a well system that incorporates one or more embodiments of an exemplary safety valve, according to the present disclosure.

FIGS. 2A and 2B illustrate cross-sectional side views of the exemplary safety valve of FIG. 1, according to one or more embodiments.

FIGS. 3A-3C illustrate enlarged cross-sectional side views of an exemplary embodiment of the piston assembly of FIG. 2A, according to one or more embodiments.

FIGS. 4A-4E illustrate enlarged progressive cross-sectional side views of another exemplary embodiment of the piston assembly of FIG. 2A, according to one or more embodiments.

DETAILED DESCRIPTION

The present disclosure relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in particular, to subsurface safety valves having a telescoping piston assembly to increase the opening force for the safety valve.

Disclosed are subsurface safety valves that use telescoping piston assemblies to move a closure device between closed and open configurations. One exemplary safety valve incorporates a piston assembly that is movably arranged within a piston bore. The piston assembly includes a piston rod and a piston that is movably arranged on the piston rod. While the piston assembly moves within the piston bore, the piston may dynamically seal an inner wall of the piston bore until engaging a bore shoulder, at which point the piston stops its movement and proceeds to dynamically seal an outer surface of the piston rod as the piston rod continues moving within the piston bore. Advantageously, the disclosed embodiments provide a large piston area for the first part of the stroke and a smaller piston areal for the remaining portions of the stroke. As a result, the larger hydraulic force is generated during the first part of the stroke, while a smaller amount of hydraulic force is generated during the remaining portions of the stroke.

Referring to FIG. 1, illustrated is a well system 100 that incorporates one or more principles of the present disclosure, according to one or more embodiments. As illustrated, the well system 100 may include a riser 102 extending from a wellhead installation 104 arranged at a sea floor 106. The riser 102 may extend, for example, to an offshore oil and gas platform (not shown). A wellbore 108 extends downward from the wellhead installation 104 through various subterranean formations 110. The wellbore 108 is depicted as being cased, but it could equally be an uncased wellbore 108, without departing from the scope of the disclosure. Although FIG. 1 depicts the well system 100 in the context of an offshore oil and gas application, it will be appreciated by those skilled in the art that the various embodiments disclosed herein are equally well suited for use in or on oil and gas rigs or service rigs, such as land-based oil and gas rigs or rigs located at any other geographical site. Thus, it should be understood that the disclosure is not limited to any particular type of well.

The well system 100 may further include a safety valve 112 interconnected with a tubing string 114 arranged within the wellbore 108 and extending from the wellhead installation 104. The tubing string 114 may be configured to communicate fluids derived from the wellbore 108 and the surrounding subterranean formations 110 to the well surface via the wellhead installation 104. A control line 116 may extend from the well surface and into the wellhead installation 104 which, in turn, conveys the control line 116 into an annulus 118 defined between the wellbore 108 and the tubing string 114. The control line 116 may extend downward within the annulus 118 and eventually become communicably coupled to the safety valve 112. As discussed in more detail below, the control line 116 may be configured to actuate the safety valve 112, for example, to maintain the safety valve 112 in an open position, or otherwise to close the safety valve 112 and thereby prevent a blowout in the event of an emergency.

The control line 116 may be a hydraulic conduit that provides hydraulic fluid pressure to the safety valve 112. In operation, hydraulic fluid may be applied to the control line 116 from a hydraulic pressure control system arranged at a remote location, such as at a production platform or a subsea control station. When properly applied, the hydraulic pressure derived from the control line 116 may be configured to open and maintain the safety valve 112 in its open position, thereby allowing production fluids to flow through the safety valve 112, through the tubing string 114, and upwards towards the rig. To move the safety valve 112 from its open position and into a closed position, the hydraulic pressure in the control line 116 may be reduced or otherwise eliminated.

Although the control line 116 is depicted in FIG. 1 as being arranged external to the tubing string 114, it will be readily appreciated by those skilled in the art that any arrangement or configuration of the control line 116 may be used to convey actuation pressure to the safety valve 112. For example, the control line 116 could be arranged internal to the tubing string 114, or otherwise formed in a sidewall of the tubing string 114. The control line 116 could extend from a remote location, such as from the earth's surface, or another location in the wellbore 108. In yet other embodiments, the pressure required to actuate the safety valve 112 may be derived from a pressure control system located downhole and communicably coupled to the control line 116 at a location.

In the following description of the representative embodiments of the disclosure, directional terms such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. In general, “above”, “upper”, “upward” and similar terms refer to a direction toward the earth's surface along the wellbore 108, and “below”, “lower”, “downward” and similar terms refer to a direction away from the earth's surface along the wellbore 108.

Referring now to FIGS. 2A and 2B, with continued reference to FIG. 1, illustrated are cross-sectional side views of an exemplary embodiment of the safety valve 112, according to one or more embodiments. In particular, the safety valve 112 is depicted in FIGS. 2A and 2B in successive sectional views, where FIG. 2A depicts an upper portion of the safety valve 112 and FIG. 2B depicts a lower portion of the safety valve 112. As illustrated, the safety valve 112 may include a housing 202 that can be coupled to the tubing string 114 at opposing ends of the housing 202 (tubing string 114 shown only in FIG. 2B).

A control line port 204 may be defined or otherwise provided in the housing 202 for connecting the control line 116 (FIG. 1) to the safety valve 112. The port 204 is shown in FIG. 2A as being plugged with a setscrew 206 or other type of plugging device. However, when the control line 116 is appropriately connected to the first port 204 the control line 116 is placed in fluid communication with a piston bore 208 and able to convey hydraulic fluid pressure thereto. The piston bore 208 may be an elongate channel or conduit defined within the housing 202 and configured to extend longitudinally along a portion of the axial length of the safety valve 112.

A piston assembly 210 may be arranged within the piston bore 208 and configured to translate axially therein. The piston assembly 210 may include a piston head 212 configured to mate with and otherwise bias an up stop 214 defined within the piston bore 208 when the piston assembly 210 is forced upwards in the direction of the control line port 204. The up stop 214 may be a radial shoulder defined within the piston bore 208 and having a reduced diameter and an axial surface configured to engage a corresponding axial surface of the piston head 212. In other embodiments, the up stop 214 may be any device or means arranged within the piston bore 208 that is configured to stop the axial movement of the piston assembly 210 as it advances upward within the piston bore and toward the control line port 204.

As illustrated, the piston assembly 210 may also include a piston rod 216 that extends longitudinally within at least a portion of the piston bore 208 and a piston 218 movably arranged about the piston rod 216. At one end, the piston rod 216 may be coupled to the piston head 212, or the piston head 212 may otherwise form an integral part thereof. At its other end (i.e., the distal end), piston rod 216 may be operatively coupled to a flow tube 220 that is movably arranged within the safety valve 112. More particularly, the piston rod 216 may be coupled to an actuator sleeve 222, and the actuator sleeve 222 may engage a biasing device 224 (e.g., a compression spring, a series of Belleville washers, or the like) arranged axially between the actuator sleeve 222 and an actuation flange 226. The actuation flange 226 forms part of the proximal end of the flow tube 220. As the actuator sleeve 222 acts on the biasing device 224 (e.g., axial force), the actuation flange 226 and the flow tube 220 correspondingly move.

Referring to FIG. 2B, the safety valve 112 may also include a valve closure device 228 that selectively opens and closes a flow passage 230 defined through the interior of the safety valve 112. The valve closure device 228 may be a flapper, as generally known to those skilled in the art. It should be noted, however, that although the safety valve 112 is depicted as being a flapper-type safety valve, those skilled in the art will readily appreciate that any type of closure device 228 might be employed, without departing from the scope of the disclosure. For example, in some embodiments, the closure device 228 could instead be a ball, a sleeve, etc.

As shown in FIG. 2B, the closure device 228 is shown in its closed position whereby the closure device 228 is able to substantially block fluid flow into and through the flow passage 230 from downhole. A torsion spring 232 biases the closure device 228 to pivot to its closed position. The piston assembly 210 is used to displace the flow tube 220 downward (i.e., to the right in FIG. 2B) to engage the closure device 228 and overcome the spring force of the torsion spring 232. When the flow tube 220 is extended to its downward position, it engages and moves the closure device 228 from its closed position to an open position (shown in phantom as dashed lines). When the flow tube 220 is displaced back upward (i.e., to the left in FIG. 2B), the torsion spring 232 is able to pivot the closure device 228 back to its closed position. Axial movement of the piston assembly 210 within the piston bore 208 will force the flow tube 220 to correspondingly move axially within the flow passage 230, and either open the closure device 228 or allow it to close, depending on its relative position.

The safety valve 112 may further define a lower chamber 234 within the housing 202. In some embodiments, the lower chamber 234 may form part of the piston bore 208, such as being an elongate extension thereof. A power spring 236, such as a coil or compression spring, may be arranged within the lower chamber 234. The power spring 236 may be configured to bias the actuation flange 226 and actuation sleeve 222 upwardly which, in turn, biases the piston assembly 210 in the same direction. Accordingly, expansion of the power spring 236 will cause the piston assembly 210 to move upwardly within the piston bore 208.

It should be noted that while the power spring 236 is depicted as a coiled compression spring, any type of biasing device may be used instead of, or in addition to, the power spring 236, without departing from the scope of the disclosure. For example, a compressed gas, such as nitrogen, with appropriate seals may be used in place of the power spring 236. In other embodiments, the compressed gas may be contained in a separate chamber and tapped when needed. In yet other embodiments, the power spring 236 may be a magnetic coupler where the biasing force is inversely proportional to displacement. For this type of biasing device, a decreasing piston area would correspond with a decreasing biasing force as the safety valve 112 stroked.

In exemplary operation, the safety valve 112 may be actuated in order to open the closure device 228. This may be accomplished by conveying hydraulic fluid under pressure (i.e., control pressure) to the control line port 204 via the control line 116 (FIG. 1). As hydraulic pressure is provided to the piston bore 208, the piston 218 assumes the hydraulic force and the piston assembly 210 is forced to move axially downward within the piston bore 208. As the piston assembly 210 moves, the piston rod 216 mechanically transfers the hydraulic force to the actuation sleeve 222 and the actuation flange 226, thereby correspondingly displacing the flow tube 220 in the downward direction. In other words, as the piston assembly 210 moves axially within the piston bore 208, the flow tube 220 correspondingly moves in the same direction. As the flow tube 220 moves downward, it engages the closure device 228, overcomes the spring force of the torsion spring 232, and thereby pivots the closure device 228 to its open position to permit fluids to enter the flow passage 230 from below.

Moreover, as the piston assembly 210 moves axially downward within the piston bore 208, the power spring 236 is compressed within the lower chamber 234 and progressively builds spring force. In at least one embodiment, the piston assembly 210 will continue its axial movement in the downward direction, and thereby continue to compress the power spring 236, until engaging a down stop 238 (FIG. 2A) arranged within the piston bore 208. A metal-to-metal seal may be created between the piston assembly 210 and the down stop 238 such that the migration of fluids (e.g., hydraulic fluids, production fluids, etc.) therethrough is generally prevented.

Upon reducing or eliminating the hydraulic pressure provided to the piston bore 208 via the control line 116, the spring force built up in the power spring 236 may be allowed to release and displace the piston assembly 210 upwards within the piston bore 208, thereby correspondingly moving the flow tube 220 in the same direction. The pressure within the safety valve 112 below the piston 218 (i.e., the section pressure) also helps move the piston assembly 210 upwards within the piston bore 208. As the flow tube 220 moves axially upwards, it will eventually move out of engagement with the closure device 228, thereby allowing the spring force of the torsion spring 232 to pivot the closure device 228 back into its closed position.

In at least one embodiment, the piston assembly 210 will continue its axial movement in the upward direction until the piston head 212 engages the up stop 214 and effectively prevents the piston assembly 210 from further upward movement. Engagement between the piston head 212 and the up stop 214 may generate a mechanical metal-to-metal seal between the two components to prevent the migration of fluids (e.g., hydraulic fluids, production fluids, etc.) therethrough.

Referring now to FIGS. 3A-3C, with continued reference to FIGS. 2A and 2B, illustrated are enlarged cross-sectional side views of an exemplary embodiment of the piston assembly 210, according to one or more embodiments. Like numerals in FIGS. 3A-3C that are used in prior figures indicate like elements and/or components that will not be described again in detail. FIGS. 3A-3C depict progressive views of the piston assembly 210 during exemplary operation. More particularly, FIG. 3A depicts the piston assembly 210 in a first position, where the safety valve 112 (FIGS. 2A-2B) is closed, as generally discussed above. FIG. 3B depicts the piston assembly 210 in an intermediate position, and FIG. 3C depicts the piston assembly 210 in a second position where the safety valve 112 has been opened or is otherwise proceeding towards being opened, as also generally discussed above.

As illustrated, the piston assembly 210 may be arranged within the piston bore 208 defined in the housing 202 of the safety valve 112 (FIGS. 2A-2B). At its proximal end (i.e., the end closest to the control line port 204), the piston rod 216 may be coupled to the piston head 212 or the piston head 212 may otherwise form an integral part thereof. The piston head 212 is shown in close contact with the up stop 214 defined within the piston bore 208 adjacent the control line port 204.

The piston rod 216 may define or otherwise provide a first or upper portion 302 a and a second or lower portion 302 b. The upper portion 302 a may exhibit a first diameter 304 a and the lower portion 302 b may exhibit a second diameter 304 b. As depicted, the first diameter 304 a may be smaller than the second diameter 304 b, and a radial shoulder 306 may be defined on the piston rod 216 to serve as a transition point between the upper and lower portions 302 a,b. In other embodiments, however, the first and second diameters 304 a,b may be substantially equal and a radial protrusion (not shown) defined on the piston rod 216 may instead serve as the transition point between the upper and lower portions 302 a,b.

The piston 218 may be movably arranged on the upper portion 302 a of the piston rod 216. More particularly, the piston 218 may be generally cylindrical and the upper portion 302 a may penetrate and otherwise extend through the piston 218, thereby allowing the piston 218 to axially translate along portions of the axial length of the upper portion 302 a. Accordingly, the piston 218 may be generally characterized as a floating piston that is movably arranged on the piston rod 216.

The piston bore 208 may provide or otherwise define at least a first or upper bore section 308 a and a second or lower bore section 308 b. The upper bore section 308 a may exhibit a first bore diameter 310 a and the lower bore section 308 b may exhibit a second bore diameter 310 b, and a bore shoulder 312 may be defined in the piston bore 208 to serve as a transition point between the upper and lower bore sections 308 a,b. As illustrated, the first bore diameter 310 a may be larger than the second bore diameter 310 b. In some embodiments, the second diameter 304 b of the lower portion 302 b of the piston rod 216 may be slightly smaller than the second bore diameter 310 b and, as a result, the lower portion 302 b of the piston rod 216 may be able to penetrate and axially translate within the lower bore section 308 b of the piston bore 208.

As illustrated, the piston 218 may be generally arranged within the upper bore section 308 a of the piston bore 208. The piston 218 may be sized such that it is able to sealingly engage the inner wall of the upper bore section 308 a and simultaneously sealingly engage the outer radial surface of the upper portion 302 a of the piston rod 216. To accomplish this, the piston 218 may include or otherwise incorporate one or more dynamic seals 314. During operation of the piston assembly 210, the dynamic seals 314 may be configured to “dynamically” seal against the inner wall of the upper bore section 308 a and the outer radial surface of the upper portion 302 a, thereby substantially preventing fluids from migrating past the piston 218 in either direction. In some embodiments, at least one of the dynamic seals 314 may be an O-ring or the like, as illustrated. In other embodiments, however, at least one of the dynamic seals 314 may be a set of v-rings or CHEVRON® packing rings, or other appropriate seal configurations (e.g., seals that are round, v-shaped, u-shaped, square, oval, t-shaped, etc.), as generally known to those skilled in the art.

In exemplary operation, hydraulic pressure or “control” pressure 316 may be introduced into the piston bore 208 via the control line 116 (FIG. 1) and associated control line port 204. Initially, as shown in FIG. 3A, the control pressure 316 acts on the piston head 212, thereby separating the piston head 212 from the up stop 214 and starting the piston assembly 210 moving in the downward direction (i.e., to the right in FIGS. 3A and 3B). Once the piston head is 212 forced out of engagement with the up stop 214, the control pressure 316 may bypass the piston head 212 and may then be able to communicate with and otherwise act on the piston 218.

Since the piston 218 sealingly engages the inner wall of the upper bore section 308 a of the piston bore 208, the upper portion 302 a of the piston rod 216 and the piston 218 cooperatively exhibit a piston area that is generally commensurate with the size of the first bore diameter 310 a. As a result, the control pressure 316 may be able to act on the full piston area of the piston 218 and piston rod 216 to move the piston assembly 210 in the downward direction. As the control pressure 316 impinges on the piston 218, the hydraulic force of the control pressure 316 on the piston 218 is transferred to the piston rod 216 via the radial shoulder 306, which prevents the piston 218 from sliding along the outer surface of the piston rod 216 and instead forces the piston rod 216 to correspondingly move downward.

Referring to FIG. 3B, the applied control pressure 316 has caused the piston assembly 210 to move to an intermediate position within the piston bore 208 where the piston 218 has contacted or otherwise come into engagement with the bore shoulder 312. The bore shoulder 312 effectively stops movement of the piston 218 with respect to the piston bore 208. However, since the piston 218 is movably coupled to the piston rod 216, and otherwise able to dynamically seal against its outer surface (i.e., the outer radial surface of the upper portion 302 a), the control pressure 316 may continue to act on the piston assembly 210 and move the piston rod 216 further downward. More particularly, the control pressure 316 may act on the piston area provided by the piston rod 216 itself (e.g., the upper portion 302 a of the piston rod 216) in order to continue the axial translation of the piston rod 216 within the piston bore 208.

Referring to FIG. 3C, the piston assembly 210 is shown in the second position, where the piston rod 216 has advanced further downward within the piston bore 208, thereby separating the radial shoulder 306 from the piston 218. As generally described above, while the piston assembly 210 moves from its first position into its second position, the piston rod 216 mechanically transfers the hydraulic force of the control pressure 316 to the flow tube 220 (FIGS. 2A-2B), thereby correspondingly displacing the flow tube 220 in the downward direction and opening the closure device 228 (FIG. 2B).

In subsurface safety valves, such as the safety valve 112 of FIGS. 2A-2B, the piston assembly 210 must overcome static friction, dynamic friction, the spring force of the power spring 236, and section pressure below the piston assembly 210 in order to initially move from the first position. Advantageously, the piston area provided by the combination of the piston rod 216 and the piston 218 is large enough to overcome such opposing forces. Individually, however, the piston area provided by the piston rod 216 is less than the piston area provided by the combination of the piston rod 216 and the piston 218. As a result, and since the force of the power spring 236 (FIGS. 2A-2B) progressively increases as it is compressed by the piston assembly 210, an increased amount of control pressure 316 would be required to produce the same amount of hydraulic force with the smaller piston area in order to move the piston assembly 210 from the intermediate position (FIG. 3B) to the second position. Accordingly, a well operator employing the piston assembly 210 may be able to use a lower maximum control line pressure (and corresponding economical and smaller control pressure equipment) in order to initially move the piston assembly 210. This could potentially provide significant savings in capital and operational expenditures for the well operator.

It will be appreciated, however, that the principles disclosed herein are not limited only to use in subsurface safety valves. Instead, the piston assembly 210 may equally be employed in any other application that requires a piston rod 216 to axially translate within a piston bore 208 and thereby move a lower mechanism (not shown) other than the flow tube 220 (FIGS. 2A-2B). For instance, the disclosed piston assembly 210 advantageously provides a large piston area for the first part of its stroke from the first position (FIG. 3A) to the intermediate position (FIG. 3B), and thereby produces a relatively large hydraulic force during this motion for a given amount of control pressure 316. Over the second part of the stroke, however, from the intermediate position (FIG. 3B) to the second position (FIG. 3C), the piston assembly 210 exhibits a smaller piston area, which equates to a smaller amount of hydraulic force that is transferred to the piston rod 216 for the same given amount of control pressure 316.

Combining these two hydraulic features may allow for a two-step operation where an increased amount of hydraulic force is required during the first part of the stroke, but a reduced amount of hydraulic force is required during the second part of the stroke. One exemplary application that may benefit from this hydraulic feature is shifting a sliding side door (not shown) from a closed position to an equalized position. For instance, during the first part of the stroke in such a case, a high load or friction must initially be overcome in order to start axial movement of the sliding side door. Once the sliding side door is moving, however, a reduced amount of hydraulic force is required to move the sliding side door to its equalized position.

Another exemplary application that may benefit from the hydraulic features of the piston assembly 210 may be a device or mechanism that incorporates one or more seals that exhibit high friction forces when static and reduced friction forces when dynamic. Yet another exemplary application may be moving an equalizing valve off its associated seat in an equalizing subsurface safety valve. Those skilled in the art will readily appreciate and recognize several other applications or scenarios where the disclosed piston assembly 210 might advantageously be employed, without departing from the scope of the present disclosure.

Other exemplary applications that may benefit from the hydraulic features of the piston assembly 210 may include any tool that requires a component to be initially sheared, broken, punctured, opened, etc. prior to moving another portion of the tool. For instance, such a tool may include one or more shear pins or shear rings that are first required to be sheared before the continuing axial force applied to the piston assembly 210 may be used to force, actuate, manipulate, or set another part of the tool. Such may be the case in a tool where the initial force shears a shear pin, and the subsequent additional stroke of the piston assembly 210 is configured to engage and expand a set of keys or lug elements in order to secure a tool in place.

Referring now to FIGS. 4A-4E, illustrated are enlarged cross-sectional side views of an exemplary piston assembly 400, according to one or more embodiments. The piston assembly 400 may be similar in some respects to the piston assembly 210 of FIGS. 3A-3C and therefore may be best understood with reference thereto, where like numerals indicate like elements and/or components that will not be described again in detail. Accordingly, in at least one embodiment, the piston assembly 400 may be used in place of the piston assembly 210 in order to actuate the safety valve 112 (FIGS. 2A-2B). FIGS. 4A-4E depict progressive views of the piston assembly 400 during exemplary operation. More particularly, FIG. 4A depicts the piston assembly 400 in a first position, where the safety valve 112 is closed, as generally discussed above, and FIG. 4E depicts the piston assembly 400 in a second or open position where the safety valve 112 has been opened or is otherwise proceeding towards being fully opened, as also generally discussed above. FIGS. 4B-4D depict the piston assembly 400 in first, second, and third intermediate positions, respectively, between the first and second positions.

As illustrated, the piston assembly 400 may be arranged within the piston bore 208 defined in the housing 202 of the safety valve 112 (FIGS. 2A-2B). The piston head 212 may be coupled to or otherwise form an integral part of the piston rod 216 at its proximal end. Moreover, similar to the piston assembly 210 of FIGS. 3A-3B, the piston rod 216 of the piston assembly 400 may define or otherwise provide the upper and lower portions 302 a,b that exhibit first and second diameters 304 a,b (shown in FIGS. 4A-4C only), respectively. Again, the first diameter 304 a may be smaller than the second diameter 304 b, with the radial shoulder 306 (labeled in FIGS. 4A-4D only) being defined on the piston rod 216 to serve as a transition point between the upper and lower portions 302 a,b. Alternatively, the first and second diameters 304 a,b may be substantially equal and a radial protrusion (not shown) may instead be defined on the piston rod 216 and serve as the transition point between the upper and lower portions 302 a,b.

Unlike the piston assembly 210, however, the piston rod 216 of the piston assembly 400 of FIGS. 4A-4E may further define or otherwise provide a piston rod neck 302 c that extends axially from the upper portion 302 a towards the piston head 212. In some embodiments, the piston head 212 may be coupled to or otherwise form an integral part of the piston rod neck 302 c of the piston rod 216. The piston rod neck 302 c may exhibit a third diameter 304 c (shown in FIGS. 4A-4C only) that is smaller than the first diameter 304 a. Moreover, an annular groove 401 may be defined on or otherwise provided by the piston rod 216 to serve as a transition point between the piston rod neck 302 c and the upper portion 302 a of the piston rod 216.

Also unlike the piston assembly 210, the piston assembly 400 of FIGS. 4A-4E may include multiple pistons 402 (shown as pistons 402 a, 402 b, and 402 c). While depicting three pistons 402 a-c, embodiments are also contemplated herein where two pistons 402 or more than two pistons 402 are used, without departing from the scope of the disclosure. The pistons 402 a-c may be similar to the piston 218 of FIGS. 3A-3C. More particularly, each piston 402 a-c may be generally cylindrical and the piston rod 216 may penetrate and otherwise extend through each piston 402 a-c, thereby allowing the pistons 402 a-c to axially translate along portions of the axial length of the upper portion 302 a.

The piston bore 208 may provide or otherwise define the upper bore section 308 a, the lower bore section 308 b, and one or more intermediate bore sections 404 (shown as a first intermediate bore section 404 a and a second intermediate bore section 404 b) that interpose the upper and lower bore sections 308 a,b. The upper bore section 308 a exhibits the first bore diameter 310 a, the lower bore section 308 b exhibits the second bore diameter 310 b, and the intermediate bore sections 404 a,b may exhibit third and fourth bore diameters 406 a,b, respectively. In at least one embodiment, as illustrated, the first bore diameter 310 a may be larger than the third bore diameter 406 a, the third bore diameter 406 a may be larger than the fourth bore diameter 406 b, and the fourth bore diameter 406 b may be larger than the second bore diameter 310 b. Accordingly, the cross-sectional bore diameters 310 a,b and 406 a,b may be configured to progressively decrease in the downward direction (i.e., to the right in FIGS. 4A-4E) through the piston bore 208.

Moreover, the piston bore 208 may define or otherwise provide a first bore shoulder 408 a, a second bore shoulder 408 b, a third bore shoulder 408 c, an upper bore relief 409 a, and an intermediate bore relief 409 b. The first bore shoulder 408 a may serve as a transition point between the upper bore section 308 a and the third bore section 404 a, the second bore shoulder 408 b may serve as a transition point between the third bore section 404 a and the fourth bore section 404 b, and the third bore shoulder 408 c may serve as a transition point between the fourth bore section 404 b and the second bore section 308 b. The upper bore relief 409 a may be an annular groove defined in the upper bore section 308 a at the first bore shoulder 408 a, and the intermediate bore relief 409 b may be an annular groove defined in the first intermediate bore section 404 a at the second bore shoulder 408 b.

The pistons 402 a-c may be sized such that each is able to sealingly engage the outer radial surface of the upper portion 302 a of the piston rod 216. Moreover, the first piston 402 a may be sized such that it is able to sealingly engage the inner wall of the upper bore section 308 a, the second piston 402 b may be sized such that it is able to sealingly engage the inner wall of the third bore section 404 a, and the third piston 402 c may be sized such that it is able to sealingly engage the inner wall of the fourth bore section 404 b. To accomplish this, the pistons 402 a-c may each include or otherwise incorporate one or more dynamic seals 314, as generally described and defined above.

In exemplary operation, control pressure 316 may be introduced into the piston bore 208 via the control line 116 (FIG. 1) and associated control line port 204. As shown in FIG. 4A, the control pressure 316 initially acts on the piston head 212, thereby separating the piston head 212 from the up stop 214 and starting the piston assembly 400 moving in the downward direction (i.e., to the right in FIGS. 4A and 4B). Once the piston head 212 is forced out of engagement with the up stop 214, the control pressure 316 may bypass the piston head 212 and may then be able to communicate with and otherwise act on the pistons 402 a-c. More particularly, the control pressure 316 may bypass the piston head 212 and act on the first piston 402 a, which sealingly engages the inner wall of the upper bore section 308 a and the outer surface of the upper portion 302 a of the piston rod 216. The upper portion 302 a of the piston rod 216 and the first piston 402 a cooperatively exhibit a first piston area that is generally commensurate with the size of the first bore diameter 310 a.

Referring to FIG. 4B, the control pressure 316 acts on the first piston area in order to move the piston assembly 400 in the downward direction. As the control pressure 316 impinges on the first piston 402 a, the hydraulic force is transferred to the piston rod 216. More particularly, since the pistons 402 a-c are cooperatively biased against the radial shoulder 306, and are therefore prevented from sliding along the outer surface of the piston rod 216, the hydraulic pressure derived from the control pressure 316 is able to be transferred through each piston 402 a-c and ultimately to the piston rod 216 via the radial shoulder 306. Upon assuming the hydraulic force, the piston rod 216 is correspondingly moved downward within the piston bore 208.

In FIG. 4B, the applied control pressure 316 has caused the piston assembly 400 to move to the first intermediate position within the piston bore 208, where the first piston 402 a has contacted or otherwise come into engagement with the first bore shoulder 408 a. The first bore shoulder 408 a effectively stops movement of the first piston 402 a with respect to the piston bore 208. However, since the pistons 402 a-c are movably coupled to the piston rod 216, and otherwise able to dynamically seal against its outer surface (i.e., the outer radial surface of the upper portion 302 a), the control pressure 316 may continue to act on the piston assembly 400 and move the piston rod 216 further downward.

As the first piston 402 a nears the upper bore relief 409 a, the second piston 402 b begins to enter the first intermediate bore 404 a. At about the same time, the dynamic seals 314 of the first piston 402 a enter the upper bore relief 409 a, thereby no longer sealingly engaging the inner wall of the upper bore section 308 a and instead allowing the control pressure 316 to migrate past the first piston 402 a and act on the second piston 402 b.

With reference now to FIG. 4C, as the piston rod 216 continues axially in the downward direction, the second and third pistons 402 b,c may correspondingly move along with the piston rod 216 within the piston bore 208 while the first piston 402 a remains at the first bore shoulder 408 a. Eventually, the annular groove 401 defined in the piston rod 216 may axially surpass the first piston 402 a and thereafter expose a first gap 410 a defined between the first piston 402 a and the reduced third diameter 304 c of the piston rod neck 302 c of the piston rod 216. Lacking a fluid tight seal between the first piston 402 a and the outer surface of the piston rod neck 302 c, the control pressure 316 is then able to also migrate past the first piston 402 a via the first gap 410 a and communicate with and otherwise act on the second piston 402 b.

The second piston 402 b sealingly engages the inner wall of the third bore section 404 a and the outer surface of the upper portion 302 a of the piston rod 216. Accordingly, the upper portion 302 a of the piston rod 216 and the second piston 402 b cooperatively exhibit a second piston area that is generally commensurate with the size of the third bore diameter 406 a. Since the third bore diameter 406 a is smaller than the first bore diameter 310 a, the second piston area is also smaller than the first piston area. As a result, an increased amount of control pressure 316 may be required to produce the same amount of hydraulic force with the second piston area in order to move the piston assembly 400 from the first intermediate position (FIG. 4B) to the second intermediate position (FIG. 4C).

As depicted in FIG. 4C, the applied control pressure 316 has caused the piston assembly 400 to move to the second intermediate position within the piston bore 208, where the second piston 402 b has contacted or otherwise come into engagement with the second bore shoulder 408 b. The second bore shoulder 408 b effectively stops movement of the second piston 402 b with respect to the piston bore 208. Again, since the pistons 402 a-c are movably coupled to the piston rod 216, and otherwise able to dynamically seal against its outer surface, further application of the control pressure 316 to the piston bore 208 may move the piston rod 216 further downward. As the second piston 402 b nears the intermediate bore relief 409 b, the third piston 402 c begins to enter the second intermediate bore 404 b. At about the same time, the dynamic seals 314 of the second piston 402 b enter the intermediate bore relief 409 b, thereby no longer sealingly engaging the inner wall of the third bore section 404 a and instead allowing the control pressure 316 to migrate past the second piston 402 b and act on the third piston 402 c.

The third piston 402 c may correspondingly move along with the piston rod 216 while the second piston 402 b remains at the second bore shoulder 408 b. The third piston 402 c will continue to be pushed by the control pressure 316 and its dynamic seals 314 will fully enter the second intermediate bore 404 b. Eventually, the annular groove 401 defined in the piston rod 216 may axially surpass the second piston 402 b and thereby expose a second gap 410 b defined between the second piston 402 b and the reduced third diameter 304 c of the piston rod neck 302 c. Lacking a fluid tight seal between the second piston 402 b and the outer surface of the piston rod neck 302 c, the control pressure 316 may then be allowed to migrate past the second piston 402 b via the second gap 410 b and communicate with and otherwise act on the third piston 402 c.

The third piston 402 c sealingly engages the inner wall of the fourth bore section 404 b and the outer surface of the upper portion 302 a of the piston rod 216. Accordingly, the upper portion 302 a of the piston rod 216 and the third piston 402 c cooperatively exhibit a third piston area that is generally commensurate with the size of the fourth bore diameter 406 b. Since the fourth bore diameter 406 b is smaller than the third bore diameter 406 a, the third piston area is also smaller than the second piston area. As a result, an increased amount of control pressure 316 may be required to produce the same amount of hydraulic force with the third piston area in order to move the piston assembly 400 from the second intermediate position (FIG. 4C) to the third intermediate position (FIG. 4D).

Referring now to FIG. 4D, the applied control pressure 316 has caused the piston assembly 400 to move to the third intermediate position within the piston bore 208 where the third piston 402 c has contacted or otherwise come into engagement with the third bore shoulder 408 c. The third bore shoulder 408 c effectively stops movement of the third piston 402 c with respect to the piston bore 208. Again, however, since the pistons 402 a-c are movably coupled to the piston rod 216, and otherwise able to dynamically seal against its outer surface, further application of the control pressure 316 may be configured to move the piston rod 216 further downward until the piston assembly 400 is placed in its second position (FIG. 4E).

As depicted in FIG. 4E, continued application of the control pressure 316 has advanced the piston rod 216 further downward within the piston bore 208, thereby separating the radial shoulder 306 from the third piston 402 c. More particularly, the control pressure 316 in FIG. 4E acts primarily on the piston area provided by the piston rod 216 itself (e.g., the upper portion 302 a of the piston rod 216) in order to continue the axial translation of the piston rod 216 within the piston bore 208. As generally described above, while the piston assembly 400 moves from its first position into its second position, the piston rod 216 may be configured to mechanically transfer the hydraulic force of the control pressure 316 to the flow tube 220 (FIGS. 2A-2B), thereby correspondingly displacing the flow tube 220 in the downward direction and opening the closure device 228 (FIG. 2B).

Accordingly, the piston assembly 400 may provide a variety of piston areas configured to progressively and cooperatively move the piston assembly 400 from the first position (FIG. 4A) to the second position (FIG. 4E). The piston area provided by the piston rod 216 itself is less than the third piston area, the third piston area is less than the second piston area, and the second piston area is less than the first piston area. As a result, and since the force of the power spring 236 (FIGS. 2A-2B) progressively increases as it is compressed by the piston assembly 400, a progressively increasing amount of control pressure 316 will be required as the piston assembly 400 moves from the first to the second positions in order to maintain the same force.

In some embodiments, the third piston 402 c may be secured or otherwise attached to the piston rod 216 with, for example, one or more snap rings, pins, mechanical fasteners, threading, etc. As will be appreciated, securing the third piston 402 c to the piston rod 216 may prove advantageous during closing of the safety valve 112 (FIGS. 2A and 2B) when the control pressure 316 is being exhausted through the piston bore 208. More particularly, if there is a delay in the power spring 236 moving the piston rod 216 or the flow tube 220 (FIGS. 2A-2B) back upward, then the section pressure below the piston assembly 400 and within the piston bore 208 may be able to move the third piston 402 c upward. Moving the third piston 402 c upward with respect to the piston rod 216 may extend the third piston 402 c past the annular groove 401, thereby allowing the section pressure to migrate past the third piston 402 c in the upward direction and travel up the control line 116 (FIG. 1).

Upon closing the safety valve 112 (FIGS. 2A-2B), the section pressure below the piston assembly 210 and the spring force of the power spring 236 (FIGS. 2A-2B) force the piston assembly back towards the up stop 214. As the piston rod 216 travels back towards the up stop 214, the radial shoulder 306 essentially serves to collect the movable pistons 402 a-c one by one. The hydrostatic head pressure and residual control pressure 316 may serve to prevent the section pressure from forcing the movable pistons 402 a-c off the seal diameter 304 a of the upper portion 302 a of the piston rod 216.

In some embodiments, the annular groove 401 in the piston rod 216 may be omitted and the third diameter 302 c of the piston rod neck 302 c may be essentially the same as the first diameter 302 a. In such embodiments, the control pressure 316 may be able to migrate past each of the first and second pistons 402 a,b via only the upper bore and intermediate bore reliefs 409 a,b, respectively. With the annular groove 401, however, the dynamic friction within the piston bore 208 may be reduced as the piston assembly 210 is stroked to the second position (e.g., the open or extended position).

Similar to the piston assembly 210 of FIGS. 3A-3C, however, the piston assembly 400 is also not limited to use in subsurface safety valves. Instead, the piston assembly 400 may equally be employed in any application requiring a piston rod 216 to axially translate within a piston bore 208 and thereby move a lower mechanism (not shown) other than the flow tube 220 (FIGS. 2A-2B). More particularly, the above-described piston assembly 400 may advantageously be used in applications requiring an increased amount of hydraulic force during the first part of the stroke, but a progressively reduced amount of hydraulic force during the remaining portions of the stroke.

As used herein, the term “dynamic seal” is used to indicate a seal that provides pressure isolation between members that have relative displacement therebetween, for example, a seal that seals against a displacing surface, or a seal carried on one member and sealing against the other member, etc. A dynamic seal may comprise a material selected from the following: elastomeric materials, non-elastomeric materials, metals, composites, rubbers, ceramics, derivatives thereof, and any combination thereof. A dynamic seal may be attached to each of the relatively displacing members, such as a bellows or a flexible membrane. Alternatively, or in addition thereto, a dynamic seal may be attached to either of the relatively displacing members, such as in the case of a floating piston.

Embodiments disclosed herein include:

A. A safety valve that includes a housing defining a piston bore configured to receive control pressure, the piston bore providing an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, the piston bore further defining a bore shoulder, and a piston assembly movably arranged within the piston bore and comprising a piston rod that extends longitudinally within the piston bore and includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion of the piston rod, wherein the piston is configured to dynamically seal an inner wall of the upper bore section when the piston assembly moves within the piston bore and dynamically seal an outer surface of the upper portion of the piston rod when the piston engages the bore shoulder and the piston rod continues moving within the piston bore.

B. A method of actuating a safety valve that includes conveying control pressure to a piston bore that provides an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, the piston bore further defining a bore shoulder, axially displacing a piston assembly arranged within the piston bore with the control pressure, the piston assembly comprising a piston rod that extends longitudinally within the piston bore and includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion of the piston rod, dynamically sealing an inner wall of the upper bore section with the piston when the piston assembly moves within the piston bore, and dynamically sealing an outer surface of the upper portion of the piston rod with the piston when the piston engages the bore shoulder and the piston rod continues moving within the piston bore.

Each of embodiments A and B may have one or more of the following additional elements in any combination: Element 1: wherein the upper portion of the piston rod exhibits a first diameter and the lower portion of the piston rod exhibits a second diameter greater than the first diameter. Element 2: wherein the piston is cylindrical and the piston rod extends through the piston. Element 3: wherein the control pressure acts on the piston and the piston rod to move the piston assembly. Element 4: wherein the piston axially biases the radial shoulder and hydraulic force derived from the control pressure acting on the piston is transferred to the piston rod via the radial shoulder. Element 5: wherein the piston is a first piston and the bore shoulder is a first bore shoulder, the safety valve further comprising an intermediate bore section defined within the piston bore and interposing the upper and lower bore sections, the intermediate bore section exhibiting a third bore diameter smaller than the first bore diameter but greater than the second bore diameter, a second bore shoulder defined in the piston bore, and a second piston movably arranged on the upper portion of the piston rod, wherein the second piston is configured to dynamically seal an inner wall of the intermediate bore section when the piston assembly moves within the piston bore and dynamically seal the outer surface of the upper portion of the piston rod when the second piston engages the second bore shoulder and the piston rod continues moving within the piston bore. Element 6: wherein the first bore shoulder provides a transition from the upper bore section to the intermediate bore section and the second bore shoulder provides a transition from the intermediate bore section to the lower bore section. Element 7: wherein the piston rod further defines a piston rod neck that extends axially from the upper portion and an annular groove defined between the upper portion and the piston rod neck, and wherein the piston rod neck exhibits a third diameter smaller than the first diameter such that a gap is formed between the first piston and the piston rod neck when the annular groove axially surpasses the first piston. Element 8: wherein the piston rod and the first piston cooperatively exhibit a first piston area, and the piston rod and the second piston cooperatively exhibit a second piston area smaller than the first piston area. Element 9: wherein the piston includes one or more dynamic seals configured to sealingly engage the inner wall of the upper bore section and the outer surface of the upper portion of the piston rod. Element 10: further comprising a flow tube operably coupled to the piston rod and movably arranged within a flow passage defined in the safety valve in response to the movement of the piston assembly, a valve closure device movable between an open position and a closed position and adapted to restrict fluid flow through the flow passage when in the closed position, wherein the flow tube is adapted to shift the valve closure device between open and closed positions, and a power spring arranged within a lower chamber defined within the housing and configured to bias the piston assembly upwardly within the piston bore.

Element 11: further comprising moving the piston assembly within the piston bore as the control pressure acts on the piston and the piston rod, and transferring hydraulic force derived from the control pressure to the piston rod when the piston engages the radial shoulder. Element 12: wherein the piston rod is operably coupled to a flow tube movably arranged within a flow passage defined in the safety valve, the method further comprising axially displacing the flow tube as the piston assembly moves within the piston bore, compressing a power spring as the piston assembly is axially displaced by the hydraulic fluid pressure, and moving a valve closure device with the flow tube from a closed position, which restricts fluid flow through the flow passage to an open position. Element 13: further comprising reducing the control pressure within the piston bore, biasing the piston assembly upwardly within the piston bore with the power spring, engaging the piston assembly on an up stop defined in the piston bore, and generating a mechanical seal between the up stop and the piston assembly. Element 14: wherein the piston is a first piston, the bore shoulder is a first bore shoulder, and wherein the piston bore further defines a second bore shoulder and an intermediate bore section that interposes the upper and lower bore sections, and wherein the piston assembly further includes a second piston movably arranged on the upper portion of the piston rod, the method further comprising dynamically sealing an inner wall of the intermediate bore section with the second piston as the piston assembly moves within the piston bore, the intermediate bore section exhibiting a third bore diameter smaller than the first bore diameter but greater than the second bore diameter, and dynamically sealing the outer surface of the upper portion of the piston rod with the second piston when the second piston engages the second bore shoulder and the piston rod continues moving within the piston bore. Element 15: wherein the piston rod and the first piston cooperatively exhibit a first piston area, and the piston rod and the second piston cooperatively exhibit a second piston area smaller than the first piston area. Element 16: wherein the piston rod further defines a piston rod neck that extends axially from the upper portion and an annular groove defined between the upper portion and the piston rod neck, the piston rod neck exhibiting a third diameter smaller than the first diameter, the method further comprising advancing the piston rod in the piston bore until the annular groove axially surpasses the first piston, allowing the control pressure to migrate past the first piston via a gap defined between the first piston and the piston rod neck, and axially displacing the piston assembly further within the piston bore as the control pressure acts on the second piston and the piston rod, wherein hydraulic force from the control pressure is transferred from the second piston to the piston rod when the second piston axially biases the radial shoulder. Element 17: further comprising separating the radial shoulder from the second piston as the control pressure acts on the piston rod and advances the piston rod further within the piston bore. Element 18: wherein conveying the control pressure to the piston bore comprises conveying hydraulic fluid to the piston bore via a control line.

Therefore, the disclosed systems and methods are well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the teachings of the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope of the present disclosure. The systems and methods illustratively disclosed herein may suitably be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted. 

What is claimed is:
 1. A safety valve, comprising: a housing defining a piston bore configured to receive control pressure, the piston bore providing an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, the piston bore further defining a bore shoulder; and a piston assembly movably arranged within the piston bore and comprising a piston rod that extends longitudinally within the piston bore and includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion of the piston rod, wherein the piston is configured to dynamically seal an inner wall of the upper bore section when the piston assembly moves within the piston bore and dynamically seal an outer surface of the upper portion of the piston rod when the piston engages the bore shoulder and the piston rod continues moving within the piston bore.
 2. The safety valve of claim 1, wherein the upper portion of the piston rod exhibits a first diameter and the lower portion of the piston rod exhibits a second diameter greater than the first diameter.
 3. The safety valve of claim 1, wherein the piston is cylindrical and the piston rod extends through the piston.
 4. The safety valve of claim 1, wherein the control pressure acts on the piston and the piston rod to move the piston assembly.
 5. The safety valve of claim 4, wherein the piston axially biases the radial shoulder and hydraulic force derived from the control pressure acting on the piston is transferred to the piston rod via the radial shoulder.
 6. The safety valve of claim 1, wherein the piston is a first piston and the bore shoulder is a first bore shoulder, the safety valve further comprising: an intermediate bore section defined within the piston bore and interposing the upper and lower bore sections, the intermediate bore section exhibiting a third bore diameter smaller than the first bore diameter but greater than the second bore diameter; a second bore shoulder defined in the piston bore; and a second piston movably arranged on the upper portion of the piston rod, wherein the second piston is configured to dynamically seal an inner wall of the intermediate bore section when the piston assembly moves within the piston bore and dynamically seal the outer surface of the upper portion of the piston rod when the second piston engages the second bore shoulder and the piston rod continues moving within the piston bore.
 7. The safety valve of claim 6, wherein the first bore shoulder provides a transition from the upper bore section to the intermediate bore section and the second bore shoulder provides a transition from the intermediate bore section to the lower bore section.
 8. The safety valve of claim 6, wherein the piston rod further defines a piston rod neck that extends axially from the upper portion and an annular groove defined between the upper portion and the piston rod neck, and wherein the piston rod neck exhibits a third diameter smaller than the first diameter such that a gap is formed between the first piston and the piston rod neck when the annular groove axially surpasses the first piston.
 9. The safety valve of claim 6, wherein the piston rod and the first piston cooperatively exhibit a first piston area, and the piston rod and the second piston cooperatively exhibit a second piston area smaller than the first piston area.
 10. The safety valve of claim 1, wherein the piston includes one or more dynamic seals configured to sealingly engage the inner wall of the upper bore section and the outer surface of the upper portion of the piston rod.
 11. The safety valve of claim 1, further comprising: a flow tube operably coupled to the piston rod and movably arranged within a flow passage defined in the safety valve in response to the movement of the piston assembly; a valve closure device movable between an open position and a closed position and adapted to restrict fluid flow through the flow passage when in the closed position, wherein the flow tube is adapted to shift the valve closure device between open and closed positions; and a power spring arranged within a lower chamber defined within the housing and configured to bias the piston assembly upwardly within the piston bore.
 12. A method of actuating a safety valve, comprising: conveying control pressure to a piston bore that provides an upper bore section having a first bore diameter and a lower bore section having a second bore diameter smaller than the first bore diameter, the piston bore further defining a bore shoulder; axially displacing a piston assembly arranged within the piston bore with the control pressure, the piston assembly comprising a piston rod that extends longitudinally within the piston bore and includes an upper portion, a lower portion, and a radial shoulder defined between the upper and lower portions, the piston assembly further comprising a piston movably arranged on the upper portion of the piston rod; dynamically sealing an inner wall of the upper bore section with the piston when the piston assembly moves within the piston bore; and dynamically sealing an outer surface of the upper portion of the piston rod with the piston when the piston engages the bore shoulder and the piston rod continues moving within the piston bore.
 13. The method of claim 12, further comprising: moving the piston assembly within the piston bore as the control pressure acts on the piston and the piston rod; and transferring hydraulic force derived from the control pressure to the piston rod when the piston engages the radial shoulder.
 14. The method of claim 12, wherein the piston rod is operably coupled to a flow tube movably arranged within a flow passage defined in the safety valve, the method further comprising: axially displacing the flow tube as the piston assembly moves within the piston bore; compressing a power spring as the piston assembly is axially displaced by the hydraulic fluid pressure; and moving a valve closure device with the flow tube from a closed position, which restricts fluid flow through the flow passage to an open position.
 15. The method of claim 14, further comprising: reducing the control pressure within the piston bore; biasing the piston assembly upwardly within the piston bore with the power spring; engaging the piston assembly on an up stop defined in the piston bore; and generating a mechanical seal between the up stop and the piston assembly.
 16. The method of claim 12, wherein the piston is a first piston, the bore shoulder is a first bore shoulder, and wherein the piston bore further defines a second bore shoulder and an intermediate bore section that interposes the upper and lower bore sections, and wherein the piston assembly further includes a second piston movably arranged on the upper portion of the piston rod, the method further comprising: dynamically sealing an inner wall of the intermediate bore section with the second piston as the piston assembly moves within the piston bore, the intermediate bore section exhibiting a third bore diameter smaller than the first bore diameter but greater than the second bore diameter; and dynamically sealing the outer surface of the upper portion of the piston rod with the second piston when the second piston engages the second bore shoulder and the piston rod continues moving within the piston bore.
 17. The method of claim 16, wherein the piston rod and the first piston cooperatively exhibit a first piston area, and the piston rod and the second piston cooperatively exhibit a second piston area smaller than the first piston area.
 18. The method of claim 16, wherein the piston rod further defines a piston rod neck that extends axially from the upper portion and an annular groove defined between the upper portion and the piston rod neck, the piston rod neck exhibiting a third diameter smaller than the first diameter, the method further comprising: advancing the piston rod in the piston bore until the annular groove axially surpasses the first piston; allowing the control pressure to migrate past the first piston via a gap defined between the first piston and the piston rod neck; and axially displacing the piston assembly further within the piston bore as the control pressure acts on the second piston and the piston rod, wherein hydraulic force from the control pressure is transferred from the second piston to the piston rod when the second piston axially biases the radial shoulder.
 19. The method of claim 18, further comprising separating the radial shoulder from the second piston as the control pressure acts on the piston rod and advances the piston rod further within the piston bore.
 20. The method of claim 12, wherein conveying the control pressure to the piston bore comprises conveying hydraulic fluid to the piston bore via a control line. 